Swift Observations of GRB 201216C

A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester) and S.R. Oates (U.Birmingham) for the Swift team

1. Introduction

At 23:07:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201216C (trigger=1013243) (Beardmore et al. GCN Circ. 29061). Swift did not slew immediately to the burst due to an observing constraint. At the time of the trigger, the initial BAT position was 116° from the Sun (7.4 hours East) and 86° from the 7%-illuminated Moon. Table 1 contains the best reported positions from Swift, and the latest XRT position can be viewed at http://www.swift.ac.uk/xrt_positions.

Izzo et al. (GCN Circ. 29066) reported the position from VLT for the optical afterglow of this GRB. Vielfaure et al. (GCN Circ. 29077) determined a redshift of 1.10 from VLT. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

Standard analysis products for this burst are available at https://gcn.gsfc.nasa.gov/swift_gnd_ana.html.

2. BAT Observations and Analysis

As reported by Ukwatta et al. (GCN Circ. 29080), the BAT ground-calculated position is RA, Dec = 16.364, 16.538 deg which is RA(J2000) = 01h05m27.4s Dec(J2000) = +16°32'16.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 18%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~ T-16 s and ends at ~T+64 s. The main peak occurs at ~T+20 s. T90 (15-350 keV) is 48.0 ± 16.0 s (estimated error including systematics).

The time-averaged spectrum from T-15.89 to T+64.11 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.43 ± 0.03. The fluence in the 15-150 keV band is 4.5 ± 0.1 x 10-5 erg cm-2. This fluence is larger than that of 99.3% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+23.61 s in the 15-150 keV band is 18.0 ± 1.1 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1013243/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 29278). We have analysed 11 ks of XRT data for GRB 201216C, from 3.0 ks to 2506.3 ks after the BAT trigger. The data comprise 466 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 29067).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.97 (+0.10, -0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.35 (+0.17, -0.16). The best-fitting absorption column is 7.7 (+1.2, -1.1) x 1021 cm-2, in excess of the Galactic value of 5.0 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.03 (+0.16, -0.15) and a best-fitting absorption column of 5.6 (+1.0, -0.9) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.2 x 10-11 (8.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.6 (+1.0, -0.9) x 1021 cm-2
Galactic foreground: 5.0 x 1020 cm-2
Excess significance: 9.3 σ
Photon index: 2.03 (+0.16, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01013243.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 201216C 2970 s after the BAT trigger (Oates and Beardmore GCN Circ. 29071). No optical afterglow consistent with the enhanced XRT position (Osborne et al. GCN Circ. 29067) or the VLT position (Izzo et al., GCN Circ. 29066) is detected in the initial UVOT exposures. Table 3 gives preliminary magnitudes using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc., 1358, 373). No correction has been made for the expected extinction in the Milky Way corresponding to a reddening of EB-V of 0.05 mag. in the direction of the GRB (Schlegel et al. 1998).

BAT light curve

Figure 1. The BAT mask-weighted light curve in the four individual and total energy bands. The units are counts s-1 illuminated-detector-1. The vertical green dash-dotted lines show the T50 interval, the vertical black dashed lines show the T90 interval, and vertical blue (orange) solid lines show the start (stop) of slews.

XRT light curve

Figure 2. The XRT light curve. Any data from a crosshatched region are not included in the fit.

RA (J2000) Dec (J2000) Error Note Reference
01h05m28.88s +16°30'58.0" 1.4" XRT-final UKSSDC
01h05m28.88s +16°30'58.0" 1.6" XRT-enhanced Osborne et al. GCN Circ. 29067
01h05m27.4s +16°32'16.4" 1.0' BAT-refined Ukwatta et al. GCN Circ. 29080

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Izzo et al. 29066 VLT afterglow candidate discovery VLT detection
Optical Jelinek et al. 29070 FRAM-ORM afterglow confirmation FRAM detection
Optical Vielfaure et al. 29077 VLT X-shooter spectroscopy and
potential high redshift of a
VHE-emitting GRB		 VLT spectroscopy
Optical Shrestha et al. 29085 Liverpool Telescope First Hour
Observations		 Liverpool Telescope light curve
Optical Belkin et al. 29210 TSHAO, Terskol, Mondy optical upper
limits		 Mondy upper limits
Optical Gokuldass et al. 29674 VIRT optical observations Virgin Island Robotic Telescope
Gamma-ray Fermi 29063 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Malacaria et al. 29073 Fermi GBM detection Fermi GBM Epeak=326±7 keV
T90=29.9 seconds
Fluence=1.41±0.06x10-4erg cm-2
(98.6 percentile for long GRBs)
Gamma-ray Nadella et al. 29074 AstroSat CZTI detection CZTI
Gamma-ray Blanch et al. 29075 MAGIC detection in very high energy
gamma rays		 MAGIC
Gamma-ray Bissaldi et al. 29076 Fermi-LAT upper limit Fermi LAT
Gamma-ray Frederiks et al. 29084 Konus-Wind detection Konus-Wind Epeak=333 (-28,+29) keV
Gamma-ray 2.at 2.UTC and Fermi-GBM 29086 No significant detection in HAWC HAWC

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
whiteFC 2970 3120 147 >20.0
white 2970 3739 344 >20.7
v 3950 4150 197 >19.1
b 3334 3534 197 >19.9
u 3129 3329 197 >19.3
m2 4154 4331 174 >19.3
w2 3745 3945 197 >19.7

Table 3. UVOT observations reported by Oates and Beardmore (GCN Circ. 29071). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

March 20, 2021